WO 2004/00198 A1 discloses a prior infrared detector which comprises a circuit block carrying an infrared sensor element, and a case accommodating therein the circuit block. The circuit block is prepared as an injection molded plastic body or so-called molded interconnect device (MID) for mounting, in addition to the infrared sensor element, various electronic components constituting a signal processing circuit which provides a detection output. In order to make the infrared detector compact, i.e., to arrange various electronic components within a limited space, the circuit block is specifically designed into a rather complicated three-dimensional shape in correspondence with the number and kinds of the electronic components, and is therefore fabricated only at an increased cost. Also for electrical connection of the infrared sensor element to the electronic components, the three-dimensional circuit block has to incorporate a correspondingly complicated circuit pattern or conductors, thereby adding an additional fabrication cost. Therefore, there is a demand for fabricating the compact circuit block or the infrared detector at a reduced cost.